Method of mounting a semiconductor device



J. E. WRIGHT ETAL 3,109,225

METHOD oF MOUNTING A sEMcoNDUcToR DEVICE:

Filed Aug. 29, 1958 Nov. 5, 1963 JDSEPH SHELLICK TERDM: E. WRIEHT 6 INVENTORS United States Patent O 3,109,225 METHOD OF MUNTNG A SEMICONDUCOR DEVICE Jerome E. .-riglit, Glen Lyon, Pa., a id .icseph Shellielr, Manviie, N5., assignors to Radio Corporation of America, a corporation et Delaware Filed Ang. 29, ESS, Ser. No. 75,iit l@ riaims. (Si. 29-1555) rl`his invention relates to improved semiconductor devices, and to improved methods of fabricating them. More particularly, the invention relates to an improved method of providing thermal dissipation in PN junctiontype semiconductor devices used in power applications.

An important class of semiconductor devices, known as transistors, usually comprises a semicondutive body having three reoions or zones of different conductivitytype separated by two PN junctions. The zones may be arranged in PNP or NPN order. Such devices with regions of alternate conductivity type separated by rectifying barriers may be formed by several methods. One of these methods is the surface alloy process, in which two pellets that induce conductivity of given type are positioned on opposing surfaces of a monocrystalline semiconductive wafer of the opposite conductivity type. The assembly is then heated so that the pellets melt and dissolve some of the wafer material. Upon cooling, the dissolved wafer material rccrystallizes in the original semiconductor lattice, but contains suicient pellet material to form a zone of the given conductivity type. At the interface between each recrystallized given conductivity type zone and the unchanged opposite conductivity type bulli of the wafer, a rectifying barrier is formed. The fused pellets become device electrodes.

In surface alloyed transistors one of the two recrystallized regions is operated as the emitter, while the other region is operated as the collector. An ohmic nonrectifying contact is made to the bulk of the wafer, which constitutes the base region of the device. When an input signal is applied between the emitter and base electrodes, the emitter region injects minority carriers into the base region. Minority carriers in a semiconductor are charge carriers of type opposite to the conductivity type of the semiconductor'. in a PNP transistor the base region is the N region, hence while the majority carriers in the base are N-type, the minority carriers in the baseregion are P-type defect electrons or holes. ln an NPN transistor the minority carriers are N-type, Le., electrons. The minority carriers are injected at low impedance, diffused through the base region, and are collected at high impedance by the collector electrode, thereby producing power gain.

The operation of semiconductor devices generates unwanted heat. As in most electrical devices, the more power handled, the more heat generated. In devices of the transistor type which contain respectively an emitter region, a rectifying barrier known as the emitter junction, a base region, a rectifying barrier known as the collector junction, and a collector region, most of the heat is generated in the collector region. The generated heat must be dissipated, since otherwise the temperature of the device rises to a level at which the thermal energy elevates electrons across the energy gap from the valence band to the conduction band, so that the device is no longer operative. In most cases, temperatures above l C. change the nature of the semiconductor surface, so that the current gain is markedly decreased even before the temperature is attained at which thermal energy renders the device inoperative. -t is desirable to keep the operating temperature of a transistor close to ambient temperature to simplify circuit stabilization. The dissipation of the generated heat is particularly important "ice the liquids generally used have been unsatisfactory because they do not provide suilcient heat dissipation, and because they often impair the electrical characteristics of the device by adversely aecting the surface of the semiconductor crystal.

A dilerent method has been used for dissipating heat from devices such as alloy junction power transistors. The transistors have been bonded to heat dissipators, or heat sinks, which are usually comparatively large metallic bodies that absorb the heat generated by the device. The heat dissipator transfers the adsorbed heat to the chassis, or directly to the atmosphere, which is the ultimate heat sink. A serious difiiculty in this method has been the production of a loW-thermal-resistance connection between the device and the metallic body which absorbs and transfers the heat. One solution of this problem utilizes an indium-to-indium cold pressure Weld between the heat sink and the alloyed indium collector electrode. However, this technique is not applicable to surface alloyed transistors in which the electrode pellets are made of materials other than indium. For example, in NPN units the electrode pellets consist predominantly of lead, with small `amounts of donors such as arsenic or antimouy. Such pellets cannot be coated with indium, since the indium forms a mechanically weak bond with lead, and furthermorer acts as an acceptor to disturb the electrical characteristics of the device. Other electrode pellet materials such as aluminum are also incapable of forming a strong bond with indium.

Attempts have been made to mount such transistors on a metallic heat sink, such a copper support, by soldering an electrode directly to the support. However, this approach has not been satisfactory, since soldering ilux and other undesirable materials are thereby spread around the electrode and its associated rectifying junction. Normally such impurities may be removed by means of an etchant, but it is riotV practicable to `etch the device when it is connected to a relatively large mass of metal. The instant application relates to a technique which may be utilized with surface alloyed semiconductor devices which have electrodes that do not contain indium, such as electrodes consisting of lead or aluminum.'

An object of this invention is to provide an improved semiconductor device suitable for power operations.'

Another object is to improve heat dissipation in junction-type semiconductor devices.

But another object of this invention is to provide an improved junction transistor having `good heat-dissipating characteristics and improved electrical characteristics.

A further object of the invention is to provide an im proved method for manufacturing semiconductor devices.

Still another object of the invention is to provide Van improved method of bonding a semiconductor device to' a heat-dissipating structure.

Yet another object of the invention is to provide an improved connection with low thermal resistance between a power transistor and a heat dissipator.

Another object of the invention `is to provide an irnproved method of mounting semiconductor devices to obtain improved cooling.

These and other objects are accomplished by the instant invention, which provides an improved room-temperature method of making a bond -with low thermal resistance between a heat d-issipator and a semiconductor device having at least one electrode comprising a material which is incapable of forming a strong bond with indium. In a preferred form of the invention, the method comprises bonding said electrode to one face of a small metal plate. The opposite face o-f the metal plate bears a layer of indium, or alterna-tively an indium layer is deposited on this face after the plate has been bonded, for example by soldering, to the electrode. Since the metal plate is relatively :small compared to the device, the assembly of device `and plate may be immersed in an :acid bath and etched to remove the flux and other impurities around the electrode. An indium layer is also deposited on a supporting base, which is preferably a thermally and electrically conductive metallic heat sink and is relatively large compared to the device. The supporting base may, for example, be a block of copper. The indium layers on the metal plate and on the supporting base are then united by pressing one in contact with the other. Preferably fresh flat surfaces are exposed on the indium layers prior to this step, and angular rotation is effected between the device-plate assembly and the supporting base while maintaining -the pressure. Since indium is weldable at relatively low temperatures and moderate pressures, molecular attraction causes the two fresh Ailat indium surfaces to coalesce and disappear. A single layer of indium is thus formed, and the device is thus bonded to the base at room temperature. As the base support is fabricated from thermally and electrically conductive material and the lmass of lthe support is relatively large compared to the mass of the device, the base not only provides mechancial support and electrical contact for the device, but also lbecomes a heat dissipator for cooling the unit. If desired, forced uid cooling means may be provided within the heat dissipator.

The invention and its features will be more ful-ly described -by the following detailed description, in conjunction with l@he drawing, wherein:

` FIGURES l-7 are sectional elevational views of a semiconductor device including a heat dissipator, showing successive steps in the fabrication of such a device in accordance -with a method of this invention;

FIGURE 8 is a schematic perspective view of apparatus used to expose a fresh flat surf-ace on an indium coating or layer which has beendeposited on a supporting base;

FIGURE 9` is Va sectional view of apparatus used to expose fa fresh flat surface on an indium-coated metal plate which thas been bonded to a junction-type semiconductor device;

FIGURE l() is a schematic perspective view of apparatus used to impart pressure and relative rotation between the semiconductor device-plate 'assembly and the supporting base which serves las a heat dissipator.

Similar reference numerals are applied to similar elements throughout the drawing.

With reference to FIGURE l, a supporting base lil is prepared from material having Igood thermal and electrical conductivity. The base is essentially a metallic block Whose mass is relatively large compared ito the transistor, and hence can serve as a heat dissipator. A suitable material for this purpose is oxygen-free, high conductivity copper. The supporting base lll may be pierced to receive terminal leads or pins. ln this example, the support lil holds a terminal lead or pin 11 -in an eyelet V12 of insulating material such as glass or the like. A second terminal lead 13 is held within a similar eyelet (not shown). A third pin l is in direct electrical and thermal contact with the base l0. A coating or layer 16 of indium is deposited at the desired site on the upper surface of the base Il). The site may, for example, be the ytop of a central boss 14. Por example, a pellet or disc of indium about lll-15 mils thick may be placed at the predetermined site and the assembly then heated on a hot plate so that the indium melts and is fused to the desired location. Preferably the indium pellet is dipped in a solution of zinc chloride before soldering it to the base. The indium tends to ball up and assume a hemispherical shape.

FIGURE 2 shows the support l() after the indium layer 16, which was originally lll-l5 mils thick, has been shaved to a thickness of about 2 4 mils.` A fresh indium surface 21 is thus exposed, which is preferably flat and parallel to the upper surface of plate I0; The upper portion of pins 11 and i3 is lgiven a right angle bend at this time.

Referring to FIGURE 3, `a. junction semiconductor device 3l containing at least one fused electrode 32 is pre-l pared by conventional methods. The incomplete device 3l, which still requires mounting, electrical connections, and casing, may be of any type kno-wn to the art, such as diodes, triodes, tetrodes and unipolar transistors. In `this example, the assembly 3l is a surface alloyed 4triode transistor such as described generally in a paper by D. A. Jenny entitled A Germanium NPN Alloy Junction Transistor in the December 1953 Proceedings of the IRE. r[the transistor assembly 3l consists of an emitter electrode pellet 34 and a similar collector electrode pellet 32 coaxially alloyed to opposite surfaces of a semiconductor wafer 36. The wafer may consist of germanium, silicon, or the like. In this example, the wafer 36 is P-type germanium, and electrode pellets 3-3 and 34 are composed of 99% lead-1% arsenic. A metal ring 38, which may for example be nickel, is ohmically soldered to the -wafer 36 around the emitter electrode 34. The ring 38 serves as the base tab,'and has a tail 39 as shown in perspective in FIGURE 10. An emitter lead 3S has a ilattened end 37 soldered over the emitter pellet 34, as shown in FIGURE 9. The collector electrode 32 is shaved to provide a flat surface 33. This may be accomplished as described below in connection with FIGURE 9. The exposed surface 33 of the lead-arsenic electrode 32 should preferably be flat and parallel to the germanium wafer 36.

Referring to FIGURE 4, one face of a metal disc 40 is soldered to the flat surface 33 of collector electrode 32. During the same heating cycle an indium pellet 41 is soldered to the opposite face of the metal disc 4d. The idisc 4t) may consist of metals such as nickel, chromium, rhodium, platinum, osmium, iridium, and palladium and alloys of these materials, which exhibit good electrical and thermal conductivity and Iare relatively inert both with respect to the semiconductor -Wafer 36 and to subsequent etchants. In this example, the disc 46* consists of nickel. Preferably disc 4G is lbut little larger than ilat surface 33 of the collector electrode 32, and has a .mass relatively small compared to device 3l.

An important advantage of this invention is that the device may now be etched to remove the excess soldering flux and other impurities which were spread over electrode 32 as a result of the previous soldering of plate 4t? to the electrode. These impurities tend to degrade the electrical characteristics of the device in an irregular manner, so that a production r-un of similar units exhibits poor and variable quality if the impurities are not removed. As explained above, if the device 31 is directly soldered to the metal support 1G, Whose mass is considerably larger than that of the device, it is impracticable to etch the unit subsequently, since the support will react with large lamounts of the etchant unless the former is made of a noble metal.. The latter expedient is obviously too expensive for com mercial units. However, plate or disc 4t? of the instant. invention has a mass relatively small compared to the mass. of the unit, and hence the entire assembly can now be immersed in an etchant Without 'dilliculty Etching may be performed as desired in either an acid or an alkaline solution by known techniques. Electrical etching may also be utilized. In this example, etching is, accomplished by immersing the assembly of unit 31 and; plate 40 withl indium pellet 41 in a bath consisting of 10% hydrogen peroxide in deionized water for about 30' minutes.

Referring to FIGURE 5, after the unit has been etched the indium pellet 41 on plate l0y is `shaved to a thickness of 2 3 mils so as to expose a fresh flat surface 50 which is preferably parallel to the nickel plate 40 and the semiconductor wafer 36.

Referring to FIGURE 6, the semiconductor assembly 31 is positioned against the support 10 so that the. fresh at surface 50 of indium layer 41 on nickel plate 40 is in contact with the fresh at surface 21 of indium layer 16 on support 10. Preferably the exposed surfaces are less than l5 minutes old, since a surface film of oxides land impurities tends to form on indium in air, and clean indium surfaces are desired for the practice of the invention. A pressure of about 3500' to 7000 grams per square inch of exposed electrode surface 33 is applied between the Vdevice 31 and the base 10. It is preferred to impart relative angular rotation between base 10 and assembly 31 while maintaining the pressure. Either the base or the assembly may be held fixed while the other is rotated. In this example, the supporting base is held stationary while the assembly 31 is rotated. The exact amount and speed of rotation is not critical. The combination of pressure and rotation of the assembly relative to the base of about 10 to 2G degrees is suficient to cause the flat surface 21 of the indium layer 15 to coalesce with the flat surface Sti of the indium layer 411. No heat is required in this operation, and a bond having low thermal resistance is thereby formed.

Referring to FIGURE 7, the base tab 38- is electrically connected to pin 11 by welding a conductive wire 72, known as a jumper wire, between pin 11 and base tab 38. The emitter electrode 34 is connected to pin 13 by weld ing jumper wire 71 between `emitter lead 35 and pin 13. External electrical `contact to the collector electrode 32 may be achieved by means of pin 15.

The final step is to case the assembly by any convenient method known to the art. In this example, a metallic cap 73 having 'a turned-out ange 74 is welded to the mounting surface of the base 10 at the flange 74. All the welding steps are sufliciently remote from the semiconductive Wafer 36 so as not to have any significant deleterious effects thereon.

The fresh clean iiat surface of the indium layer or pellet 16 on base 1@ and the indium pellet 41 `on plate 40 may be exposed by Iany convenient method. For example, the indium layer and the indium pellet may be shaved by hand with a razor blade. The assembly may then be manually mounted on the supporting base. However, it is preferred to use apparatus such as shown in FIGURES 8, 9 and 10 for mass production of semiconductor junction devices in accordance with this invention.

Referring to FIGURE 8, the base 10 bearing the indium pellet or disc 16 uppenmost is positioned on the work table 82 of the shaving apparatus S1. Positioning is accomplished by inserting pins 11, 13 `and 15 into recesses (not shown) in the work table I82. Adjacent the work table 82 of the apparatus is a raised portion 83 containing a slot 84, lwhich holds and directs the lcutting tool 85 over the indium disc 16. A single stroke of the cutting tool 35 across the indium disc 16 shaves off the upper portion of the disc 16, leaving a'layer of indium about 2-4 mils thick, and exposing an indium surface 21 which is fresh, flat, and parallel to the face of supporting base 10. The same apparatus is used to slice indium pellet 41 on plate 40 of the device, and thus expose an indium surface 50 which is fresh, at, and parallel to plate 40 and wafer 36.

Referring to FIGURE 9, a ilat surface 33 on the leadarsenic fused electrode 32 of the semiconductor assembly 31 is prepared by means of slicing apparatus 91. The device 31 is pressed in a recess 92 of a pivoted horizontal plate 93. The recess is deep enough to permit a portion of the alloyed electrode -32 to protrude below the layer surface of the pivoted plate 93. A horizontal blade 94 6 is fixed just below the lower surface lof the pivoted plate 93. On swinging the plate 93 :against the blade 94, the protruding portion of the collector electrode 32 is sliced away, and `a flat surface 33 is thereby exposed.

Referring to FIGURE l0, the apparatus 101 for mounting the semiconductor assembly 31 on the base 10 comprises a spring loaded pressure pin 102, and a drive pin 103, which are held in the lower end of an adjustable, vertically mounted, rotatable metal sleeve 104. The pressure pin 102 is coaxial with the metal sleeve 104, while the drive pin 103 is olfset from the axis of sleeve 104. Attached to the sleeve 104 is a horizontal handle 10S which controls the rotation and height of the sleeve. In this example, the spring loaded pressure pin 102 is adjusted to exert a pressure of 5000 grams per square inch. The amount of rotation of the handle 105 is preset by two positive stops. In this example, the handle is preset to rotate approximately 18 degrees. The plate 10 is positioned in the apparatus 81 so that the freshly exposed surface 21 of indium layer 16 is directly below the pressure pin 102. After the fresh at surface 50 of indium layer 41 on plate 40 of assembly 31 has been exposed in the apparatus shown in FIGURE 8, the assembly 31 is placed on base 10 so that the fresh flat indium surface 50 contacts the fresh surface 21 of the indium layer 16. The sleeve 104 is then lowered, so that the pressure pin 102 is forced against the flat portion 37 of the emitter lead v 35. A pressure of about 5000 grams per square inch is thus exerted between the flat indium-covered surface 50 and the flat surface 21 of the indium layer. At the same time, the handle 105 is rotated approximately 18 degrees, This revolves the drive pin 103 against the tail 39 of the base tab 3S, and hence causes the assembly 31 to rotate about 18 degrees. A bond is thus formed, in which the fresh surface 50 of indium layer 41 on plate 40 is coalesced with the fresh surface 21 of the indium layer 16 on the base 10. The bond is mechanically strong and has low thermal reseistance.

To provide still further cooling, the heat dissipator 10 may be provided with a continuous channel (not shown), throughout which a iiuid cooling means may be circulated during operation ofthe device.

It will be understood that while the invention has been described by way of example in connection with a power transistor of the triode type, it is by no means limited by such application. Other types of semiconductor assemblies known to the art, and having a greater or smaller number of electrodes and PN junctions, may be similarly treated to provide good thermal dissipation. For example, a single junction semiconductor assembly known as a diode may be prepared by alloying an indium pellet to a surface of a wafer of N-conductivity type germanium. The pellet may then be shaved to expose a fresh flat indium surface, and mounted on a heat dissipating base by the method described above.

Although the invention has been described with reference to a germanium unit, it will be understood that the invention may be practiced with surface alloyed devices in which the semiconductor wafer is composed of other semiconductors such as gallium arsenide, indium phosphide, and the like. For example, the invention may be practiced with surface alloyed silicon devices such as described generally in a paper by H. Nelson entitled A Silicon N-P-N Junction Transistor by the Alloy Process, Transistors I, published March 1956, by RCA Laboratories, Princeton, New Jersey. It will also be understood that the invention is equally applicable to PNP units, for example units in which the alloyed electrodesy consist of aluminum or aluminum alloys.

There has thus been described a novel structure and arrangement for etciently dissipating heat in power semiconductor devices. The effect is obtained by making bond having low thermal resistance between a device electrode where heat is generated, and a relatively large Volume 7 heat dissipator. Measurements indicate that thethermal resistance of the bond made in accordance with this invention is about 4 C. per Watt dissipated.

What is claimed is:

l. The method of mounting a semiconductor device including a lead-containing electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys of diese metals, depositing a coating of indium on the opposite face of said plate, exposing a fresh ilat surface of said indium coating, depositing a layer of indium on a supporting member, exposing a fresh flat surface of said indium layer, contacting said fresh indium surface on said plate to said fresh indium surface on said supporting member, pressing the device-plate assembly to said supporting member, and rotating said device-plate assembly While maintaining pressure between said device-plate assembly and said supporting member to coalesce said indium surfaces.

2. The method of mounting a surface alloyed semiconductor device including at least one fused lead-containing electrode, said electrode being incapable of forming a strong bond with indium, comprising the steps of bonding said electrode directly to one face of a nickel plate, said plate being relatively small compared to said device, depositing a coating of indium on the opposite face of said plate, exposing a fresh llat surface of said indium coating, depositing a layer of indium on a supporting base, exposing a fresh at surface of said indium layer, and pressing said exposed indium surface on said opposite face of said plate against said exposed indium surface of said base while effecting relative angular rotation between said ybase and said plate to coalesce said indium surfaces.

3. The method of mounting a semiconductor device including a lead-containing electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys of these metals, depositing a coating of indium on the opposite face of said plate, exposing a fresh flat surface of said indium coating, depositing a layer of indium on a supporting base, exposing a fresh flat surface of said indium layer, positioning the device-plate assembly on said base so as to contact said exposed indium surface on said plate to said exposed indium surface on said base, and pressing said device-plate assembly to said base plate While imparting angular rotation to said device-plate assembly so as to coalesce Said indium surfaces.

4. The method of mounting a semiconductor device including a lead-contm'ning electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from lthe group consisting of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys `of these metals, depositing a coating of indium Ion the opposite surface of said plate, shaving said coating to expose a fresh ilat indium surface, depositing a layer of indium on a supporting base, shaving said layer to expose a fresh flat indium surface, positioning the device-plate assembly `on saidA base so as to contact said fresh indium surfaces, and pressing said deviceplate assembly to said base While imparting suilicient angular -rotation to said device-plate assembly so as to coalesce said indium surfaces and bond said device-plate assembly to said base.

5, The method of mounting a semiconductor device including a lead-containing electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, chromium, rhodium, platinum, os-

mium, ir-idium, palladium, and alloys of these metals, depositing a coating of indium on the opposite face of said plate, shaving said coating to expose a fresh flat indium surface, depositing a layer of indium on -a predetermined portion of a thermally conductive base, shaving said layer to expose a fresh flat indium surface, contacting said exposed indium surface on said plate to said exposed indium surface on said base, and pressing the device-plate assembly to said base While imparting suflicient angular rotation to said device-plate assembly so as to coalesce said indium surfaces and bond said deviceplate assembly to said base.

6. The method of mounting a surface alloyed lsemiconductor device including at least one lead-containing fused electrode, comprising the steps of bonding said electrode directly :to one face of a metal plate, said plate being relatively small 1compared to said device, said metal being selected from the group consisting of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys of these metals, depositing a coating of indium on the opposite face of said plate, shaving said coating to expose a fresh flat indium surface, depositing an indium layer about 10-15 mils thick on a thermally conductive base, shaving said indium layer to .a thickness of about 2-4 mils so as to expose a fresh flat indium surface, contacting saidexposed indium surface on said plate to said exposed indium surface yon said base, and pressing the device-plate assembly to said base While imparting suicient langular rotation to said device-plate assembly so as to coalesce said indium surfaces and bond said deviceplate assembly to said supporting base. n

7. The method of mounting a surface alloyed semiconductor device including at least one fused lead-containing electrode, comprising the steps yo-f bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, chro-` mium, rhodium, platinum, osmium, iridium, palladium, and alloys of these metals, depositing a coating of indium on the opposite face `of said plate, shaving said coating to exp-ose a fnesh flat indium surface, depositing an indium layer 'about 10-15 mils thick on a thermally conductive base, shaving said layer to a thickness of about 2-4 milsy so as to expose a fresh flat indium surface, contacting said exposed indium surface on said plate to said exposed indium surface on said base, pressing the device-plate assembly to said base with a pressure of about 350010 7000 grams per square inch of indium surface on said plate, and rotating said device-plate assembly Iabout l0 to 20 While maintaining said pressure so as to coalesce said indium surfaces and bond said device-plate assembly to said base.

8. The method of mounting a surface alloyed semiconduct-or device including at least one lead-containing fused electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting `of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys of these metals, depositing a coating yof indium on the opposite face of said plate, shaving said coating to expose a fresi flat indium surface, depositing an indium layer about 10-15 mils thick on a thermally and electrically conduct-ive base, shaving said layer to a thickness of about 2-4 mils so as to expose a fresh flat indium surface, contacting said exposed indium surface on said plate 'to said exposed indium surface on said base, pressing the device-plate assembly to said base with a pressure of about 500() grams per square inch of indium surface on said plate, fand rotating said device-plate assembly about i8 While maintaining said pressure so 4as to coalesce said indium surfaces and bond said device-plate assembly to said base.

9. The method of mounting a surface ralloyed semiconductor device including at least one lead-containing fused electrode, comprising the steps of bonding said electrode directly to one face of la metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, claroh mium, rhodium, platinum, osmiuml, iridium, palladium, and alloys of these metals, depositing a layer of indium on the opposite face of said plate, shaving said layer to expose a fresh at indium surface, depositing a layer of indium on a supporting base, shaving said indium layer on said base to expose a fresh at indium surface, and uniting said indium `layers by pressing one in Contact with the other.

10. The method of making a surface alloyed semiconductor device including at least one lead-containing fused electrode, comprising the steps of bonding said electrode directly to one face of a metal plate, said plate being relatively small compared to said device, said metal being selected from the group consisting of nickel, chromium, rhodium, platinum, osmium, iridium, palladium, and alloys of these metals, depositing a coating of said indium on the opposite face of said plate, shaving said coating -to expose a fresh flat indium surface, depositing an indium layer yabout 10-15 mils thick on a thermally and electrically conductive base, shaving said layer to a thickness of about 2-4 mils so as to expose a fresh flat indium surface, contacting said fresh surface on said plate to said fresh surface on said base within 15 minutes after said surfaces have been exposed, pressing the device-plate 10 assembly to said base with a pressure of about 5000 grams per square -inch of indium surface -on said plate, and rotating said device-plate assembly about 18 while maintaining said pressure so as to coalesce said fresh indium surfaces and bond said device-plate assembly to said base.

References Cited in the iile of this patent UNITED STATES PATENTS 1,624,501 Nelson Apr. 12, 1927 1,661,448 Taylor Mar. 6, 1928 2,443,870 Reynolds June 22, 1948 2,662,500 For-t etal Dec. 15, 1953 2,671,746 Brew Mar. 9, 1954 2,707,889 Sowter May 10, 1955 2,731,704 Spanos Jan. 24, 1956 2,754,238 Arenberg July 10, 1956 2,790,656 Cook Apr. 30, 1957 2,820,932 Looney Jan. 21, 1958 2,854,612 Zar-atkiewicz Sept. 30, 1958 2,907,935 Nagorsen Oct. 6, 1959 2,984,774 Race May 16, 1961 2,985,550 Anderson May 23, 1961 2,990,502 Willemse et al June 27, 1961 3,002,271 Thornton Oct. 3, 1961 OTHER REFERENCES Pressure Welding, article in The Welding Journal, August 1951, pp. 728730. 

9. THE METHOD OF MOUNTING A SURFACE ALLOYED SEMICONDUCTOR DEVICE INCLUDING AT LEAST ONE LEAD-CONTAINING FUSED ELECTRODE, COMPRISING THE STEPS OF BONDING SAID ELECTRODE DIRECTLY TO ONE FACE OF A METAL PLATE, SAID PLATE BEING RELATIVELY SMALL COMPARED TO SAID DEVICE, SAID METAL BEING SELECTED FROM THE GROUP CONSISTING OF NICKEL, CHROMIUM, RHODIUM, PLATINUM, OSMIUM, IRIDIUM, PALLADIUM, AND ALLOYS OF THESE METALS, DEPOSITING A LAYER OF INDIUM ON THE OPPOSITE FACE OF SAID PLATE, SHAVING SAID LAYER TO EXPOSE A FRESH FLAT INDIUM SURFACE, DEPOSITING A LAYER OF INDIUM ON A SUPPORTING BASE, SHAVING SAID INDIUM LAYER ON SAID BASE TO EXPOSE A FRESH FLAT INDIUM SURFACE, AND UNITING SAID INDIUM LAYERS BY PRESSING ONE IN CONTACT WITH THE OTHER. 